Bleaching of mechanical wood pulp with boranes



2,960,427 Patented Nov. 15, 1960 BLEACHIING OF NIECHANICAL WOOD PULP WITH BORANES Talivaldis Berzins, Wilmington, DeL, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del a corporation of Delaware No Drawing. Filed Sept. '3, 1958, Ser. No. 758,682

Claims. (Cl. 162-7) This invention relates to an improved process for bleaching wood pulp and, more particularly, this invention relates to a process wherein mechanically-integrated wood-pulp slurries are eifectively bleached when contacted with novel bleaching agents under a wide range of conditions.

The term "mechanically-disintegrated wood pulp includes within its meaning groundwood pulp, obtained by grinding logs to fibers, and semi-chemical wood pulp. The latter-referred-to pulp is obtained by reducing logs to chips, treating the chips at elevated temperatures and pressure with chemicals and thereafter grinding the chips to fibers. Both of these pulps are particularly difficult to bleach, since they have a relatively high lignin content.

Pulps of the above type have been bleached with chemicals that are either oxidizing or reducing agents. In the latter category are sodium bisulfite and sodium hydrosulfite which have been used in processes that suffer from a considerable number of disadvantages. Processes for treating pulp with sodium bisulfite require lengthy reaction times, and the increase in brightness obtained is small (about 4 points G.E.). Processes which use hydrosulfite will enable greater increases in brightness within a shorter period of time, but the process must be carefully controlled with respect to pulp consistency and PH. In addition, the chemicals used decompose readily in air at high temperatures; air must be eliminated during the bleaching process. Pulp slurries containing hydrosulfites are extremely corrosive. Furthermore, pulps bleached with hydrosulfites exhibit limited brightness stability, los ing considerable brightness on standing or on exposure to light and heat.

It is an object of the present invention to provide a process for bleaching mechanically-disintegrated pulp using novel bleaching agents, which agents may be employed over a wide pH range.

It is another object of the present invention to provide a bleaching process wherein the pulp slurry being treated is not excessively corrosive.

It is still another object of the present invention to provide a bleaching process wherein the bleaching agent in the pulp slurry is relatively stable under operating conditions.

These and other objects are accomplished by employing amine-boranes as bleaching agents.

tertiary amine-boranes are those of lower molecular Weight. In other words, ammonia and derivatives of ammonia having lower alkyl radicals (1-5 C atoms) as substituents are most suitable as bleaching agents, al-

2 though amine-boranes of higher molecular weight are operative.

It is a common characteristic of those compounds, wherein borane (EH is attached at the nitrogen atom, to decompose under the conditions outlined herein releasing the borane adduct which is the actual bleaching agent.

In carrying out the bleaching process, the groundwood pulp of proper consistency and pH is mixed thoroughly with the required amount of bleaching solution to produce a pulp slurry of the desired consistency and bleaching agent concentration. This pulp slurry is then maintained at the required temperature untilthe pulp is bleached to the desired brightness. In bleaching pulp with amineboranes, such standard operations as mixing and transfer of pulp, adjustment of pulp consistency and pH, heating of pulp slurries, etc., are performed by the conventional methods known in the art.

Unlike prior art reduction bleach processes, the consistency of the pulp suitable for use in the present invention is not limited. For instance, the use of sodium hydrosulfite is not possible in slurries having consistencies higher than 6%, since air becomes entrapped in the slurries and destroys the bleaching agent. The consistency of pulp slurries are not at all critical when amineboranes are used. Slurries of low (1-6%), medium (7-15%) and even high (20% or higher) consistencies may be effectively bleached with amine-boranes. Of course, agitation should be provided to insure proper contact between the fibers and the bleaching agent.

Bleaching with amine-boranes is operative from room temperature to the boiling point of Water. Bleaching can be done under pressure at a higher temperature than 100 C. As the temperature of the pulp slurry is raised, the rate of bleaching increases. As a practical matter,

' bleaching in the temperature range between 40-80 C.

is preferred.

Amine-boranes may be employed as bleaching agents either for acid or alkaline pulp slurries. The pH range in which amine-boranes can be used as bleaching agents varies from about 3-14, the preferred range being 5-11. To make pH adjustments, practically any material pres ently used in reduction pulp bleaching may also be employed in the presently described process. For instance, sulfuric acid, sodium hydroxide and sodium silicate may be used to adjust the pH. Since amine-boranes are oxidized rapidly by water in a strongly acid medium, bleaching with amine-boranes becomes ineificient when the pH of the pulp slurry is lower than about 3.0.

Bleaching solutions are prepared by dissolving arnineboranes in Water. Water solutions of amine-boranes are generally neutral or slightly basic (pH 7.0-8.5). If alkaline bleaching solutions are employed, the required mnount of a base (e.g., NaOH) and/or an alkaline buffer (e.g.,'sodium silicate) is added to the amine-borane solution. Such additives as sodium tripolyphosp-hate, ethylenediaminetetraacetic acid or its salts, sodium silicate, magnesium sulfate, 'etc., which are commonly used in groundwood bleaching, may also be added, if desired, to the amine-borane bleaching solution.

The rate of bleaching and the final brightness of the pulp increase rapidly with increasing concentrations of amine-borane. The borane substituent in an arnineborane is the active bleaching agent. It has been found that an amount of at least 0.04% by Weight of borane should be used, and more preferably, 0.08% will effectively bleach the pulp. The amount of borane is based on the weight of moisture-free pulp content (conventionally known as pulp that is oven-dried at 0.). In the instance of isopropylamine-borane the amount of that crzngound required to furnish 0.08% borane is about 0. 2 o.

There is no upper limit of the amount of borane which can be used to bleach pulp; preferred amounts are 0.08- 0.5% and amounts of the particular amine-borane required to furnish borane within this range can be readily determined by one skilled in the art.

Bleaching of mechanically-disintegrated pulp with amine-boranes depends on a number of interrelated variables, each of which has a decided effect on the other. These variables include amine-borane concentration, pulp consistency and pH, bleaching temperature and time. The below are illustrative examples of the effect of the aforementioned variables on the groundwood bleaching with amine-boranes.

Nineteen grams of spruce groundwood, containing 3 g. moisture-free pulp and having a pH of 5.5, were thoroughly mbred with bleaching solutions containing the given amounts of chemicals and bleached at different temperatures for various periods of time. After bleaching the pulp was slurried in 800 ml. of water, the pH adjusted to 56,-and handsheets of- 18.3 cm. in diameter were prepared. The handsheets were dried at room temperature, and their reflectances (brightness) were measured by the Hunter reflectometer using a blue filter. The results of groundwood bleaching with isopropylamineborane, (CH CHNH :BH are summarized in Table I. It'is seen from Table I that the brightness of bleached pulp depends strongly on the isopropylamine-borane concentration. Practically no bleaching is obtained when less than 0.5% of isopropylamine-borane is used. In fact, some darkening of pulp is observed when less than 0.25% of that compound is employed. In the concentration range from 12%, isopropylamine-borane produces brightness which is comparable to that obtained with hydrogen peroxide or hydrosulfites in commercial bleaching processes. In general, alkaline bleaching solutions containing amine-boranes produce slightly brighter pulp than an acid bleaching solution.

The rate of bleaching increases rapidly with increasing temperatures; e.g., the increase in brightness obtained at 25 C. is only one-third of that obtained at 98 C. during the same period of time. At a constant temperature approximately 75% of the total increase in bright ness is obtained in the first thirty minutes. Similar results were obtained when groundwood pulp was bleached with isopropylamine-borane at 5% consistency.

The following examples show that a variety of amineboranes are effective bleaching agents for groundwood pulp. The results summarized below were obtained by bleaching groundwood at 12% consistency and pH 56 for one hour.

Bleaching Agent Temperature ness of Bleached Pulp Unbleached- 60. 7 Ammonia-borane (H NIBHa) 60 70.1 htethylamine-borane GHaNHzIBHs 60 0 6G. Dimethylaminc-borane (CH HN:BH 60 0 64. 1 60 0 62. 2 Diethylaminc borane (CzHli)2HN.BH3 8 m1 61.8 Trimethylamine borane (011mb. .BHK 100 0 63' 6 TABLE I Bleaching of groundwood pulp with isopropylamineborane Hunter Brightness Percent Isopropylamine- Temper- (Blue Light) bora-ne (Based on Moisture attire, Time.

Free Pulp Content) Minutes Alkaline Acid Bleach Bleach Unbleached 60. 1 61. 5

EFFECT OF TEMPERATURE 1 Consistency: 12%.

2 Alkaline bleach contained, in addition to the given amount 01 IPAB the following chemicals: MgSO -7H O0.05%, sodium silicate (40 1%., weight ratio of Na O to SlOz=1Z3.25)5.0%, and NaOH-1.23%, based on moisture tree weight of pulp.

3 Acid bleach contained only IPAB (pH 5.0-6.0).

To produce pulp of high brightness (70-80 Hunter brightness units), a two-step bleaching process can be used involving peroxides and amine-boranes as bleaching agents. In the first step a brightness in the range of 6373 G.E. units is developed by bleaching with peroxides. The second step involves reductive bleaching with amine-boranes, resulting in an additional increase in brightness (5-7 units).

Any conventional peroxide prebleach treatment may be used provided that the pulp slurry is washed to remove the peroxide residues.

After bleaching withH O the pulp slurry was neutralized and residual H 0 decomposed by adding sodium metabisulfite. The H O -bleached pulp was then bleached at 12% consistency with amine-boranes. The results obtained with isopropylamine-borane and dimethylamineborane are summarized in Table II below. The regular hydrogen peroxide bleaching increased the brightness of groundwood about 10 units, and the second bleaching with IPAB gave an additional increase of about 5-6 units, bringing the final brightness up to 78. Under comparable conditions bleaching with DMAB gave smaller increase in brightness. The order of bleaching steps may be reversed, but this generally produces pulps of somewhat lower brightness. Bleaching of peroxideprebleached pulps with amine-boranes was in general more eflicient under acid bleaching conditions (pH: 5-6) and the amount of borane as low as 0.04% by weight, based on moisture-free pulp content, produced a noticeable bleaching effect.

1 Composition of alkaline and acid bleaches is given in Table I. Consistency: 12%.

The bleaching of semi-chemical wood pulp using isopropylamine-borane is shown in Table III.

TABLE III Bleaching of chip groundwood with isopropylamz'neborane Stock pulp: hardwood (a mixture of beech, birch and maple) chip groundwood. Consistency: 16.2%.

This pulp was bleached by adding water solution of isopropylamine-borane. The consistency of the pulp durmg bleaching was 12%.

RESULTS Percent Isopropylamine-borane Hunter (Based on Moisture Free Pulp Temper- Time, Brightness Content) ature, 0. min. (Blue Light) Unbleached- 45. 7 2%- 95-98 30 57. 1 1% 95-98 30 53. 8 2%- 70 60 54. 1%- 70 60 52. 5 Prebleached with H303 1 61. 4 1% 95-98 30 70. 4 1% 70 6O 68. 9 2% 70 60 68. 9

1 Prebleaching chip groundwood with H2O; was done with the same bleach composition as for ordinary groundwood. Consistency: 12%. Temperature 70 C. Bleaching Time: 2 hours.

Mechanically-disintegrated wood pulps bleached with amine-boranes exhibit greater stability in brightness than pulps bleached with conventional bleaching agents. For instance, accelerated heat aging tests (exposure at 105 C. for one hour) show that the loss of brightness in isopropylamine-borane bleached pulp varies from 1.0-1.6 units as measured on the Hunter reflectometer with the blue filter. The results obtained in the Hunter reflectometer with a blue filter approximate the brightness as recorded by the G.E. reflection meter. In comparison, the loss in peroxide-bleached pulp under identical conditions is reported to be 1.5-3.0 G.E. units.

The embodiments of the invention in which an ex- 6 elusive property or privilege is claimed are defined as follows:

1. In a process for bleaching mechanically-disintegrated wood pulp by reduction, the step of contacting said pulp at pH 3-14 with at least 0.04% by weight of borane, said amount of borane being based on moisture-free pulp content and supplied by a compound selected from the group consisting of Rz wherein R R and R are selected from the group consisting of H and lower alkyl radicals having 1-5 C. atoms.

2. In a process for bleaching mechanically-disintegrated wood pulp by reduction, the step of contacting said pulp at pH 5-11 with at least 0.04% by weight of borane, said amount of borane being based on moisture-free pulp content and supplied by a compound selected from the group consisting of 11/ wherein R R and R are selected from the group consisting of H and lower alkyl radicals having 1-5 C. atoms.

3. In a process for bleaching mechanically-disintegrated wood pulp by reduction, the step of contacting said pulp at pH 3-14 with at least 0.04% by weight of borane, said amount of borane being based on moisture-free pulp content and supplied by the compound, ammonia-borane.

4. In a process for bleaching mechethically-disintegrated wood pulp by reduction, the step of contacting said pulp at pH 3-14 with at least 0.04% by weight of borane, said amount of bo-rane being based on moisture-free pulp content and supp-lied by the compound, isopropylamineborane.

5. In the bleeching of mechanically-disintegrated wood pulp wherein wood-pulp slurries are bleached with peroxides under highly alkaline conditions and thereafter neutralized and purged of residual peroxide, the combination of said pretreatment with the step of contacting said peroxide-bleached wood pulp having a pH 3-14 with at least 0.04% by weight of borane, said amount of borane being based on moisture-free pulp content and supplied by a compound selected from the group consisting of R: wherein R ,R and R are selected from the group consisting of H and lower alkyl radicals having from 1-5 C. atoms.

References Cited in the file of this patent UNITED STATES PATENTS Kise et a1. Dec. 2, 1958 Jullander Aug. 4, 1959 OTHER REFERENCES UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No.o 2,96QA27 November 15, 1960 Talivaldis Berzins It is hereby certified'that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 17, for "mechanically=-=inteq rated read mechanically-=disintegrated line 39, for "PH" read pH cglumn 4, TABLE I, second line of footnote 2thereof, for "MgSO "71-120" read MgSO4 7H2O line 54, for f'MgSOgflI-IgO" read MgSO 7H O column 5, TABLE 11, column 1, line 1 thereof, for "H 0 read H 0 column 6, line 39, for "loleeching read bleaching =0 Signed and sealed this. 27th day of June 1961.,

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents 

1. IN A PROCESS FOR BLEACHING MECHANICALLY-DISINTEGRATED WOOD PULP BY REDUCTION, THE STEP OF CONTACTING SAID PULP AT PH 3-14 WITH AT LEAST 0.04% BY WEIGHT OF BORANE, SAID AMOUNT OF BORANE BEING BASED ON MOISTURE-FREE PULP CONTENT AND SUPPLIED BY A COMPOUND SELECTED FROM THE GROUP CONSISTING OF 